Ester-based solvent systems for printable organic light-emitting diode ink formulations

ABSTRACT

Ink compositions for forming active layers in an organic light-emitting diode are provided. Also provided are methods of forming active layers of an OLED using the ink compositions. The ink compositions comprise a solvent system in which a substantial majority of the solvent is comprised of one or more ester compounds.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 15/424,121 filed on Feb. 3, 2017. U.S. patentapplication Ser. No. 15/424,121 is a divisional application of U.S.patent application Ser. No. 14/012,315, filed on Aug. 28, 2013, whichissued as U.S. Pat. No. 9,640,772 on May 2, 2017. U.S. patentapplication Ser. No. 14/012,315 claims benefit of U.S. ProvisionalPatent Application No. 61/865,087 filed on Aug. 12, 2013. Allapplications cited in this section are incorporated herein by referencein their entirety.

SUMMARY

Ink compositions for forming active layers in an OLED are provided. Alsoprovided are methods of forming active layers of an OLED using the inkcompositions. Hole transport layers and light-emitting layers areexamples of the types of active layers that can be formed using thepresent ink compositions. A substantially majority of the solvent in theester-based solvent system comprises ester solvents, such as alkyloctanoates, alkyl sebacates or combinations of such esters with otherester solvents.

One embodiment of a method of forming a light-emitting layer for anorganic light-emitting diode comprises the steps of: (a) forming a layerof an ink composition in an organic light-emitting pixel cell, the pixelcell comprising an emission region defined by a pixel bank, the inkcomposition comprising an organic electroluminescent material dissolvedin an ester-based solvent system comprising at least one ester, theester-based solvent system having a boiling point of at least 300° C.and a surface tension at 23° C. in the range from about 26 dyne/cm toabout 33 dyne/cm; and (b) allowing the solvent from the solvent systemto evaporate, whereby the light-emitting layer is formed. In someembodiments, the ester-based solvent system for a light-emitting layercomprises, consists of, or consists essentially of diethyl sebacate. Inother embodiments, the ester-based solvent system for a light-emittinglayer comprises, consists of, or consists essentially of octyloctanoate. In still other embodiments, the ester-based solvent systemfor a light-emitting layer comprises, consists of, or consistsessentially of a mixture of octyl octanoate with at least one of diallyphthalate and isononyl isononate.

One embodiment of a method of forming a hole transporting layer for anorganic light-emitting diode comprises the steps of: (a) forming a layerof an ink composition in a pixel cell, the pixel cell comprising anemission region defined by a pixel bank, the ink composition comprisingan hole transporting material comprising a crosslinking polymer, whereinthe hole transporting material is dissolved in an ester-based solventsystem comprising at least one ester, the ester-based solvent systemhaving a boiling point of at least 300° C. and a surface tension at 23°C. in the range from about 25 dyne/cm to about 32 dyne/cm; and (b)allowing the solvent from the solvent system to evaporate, whereby thehole transporting layer is formed. In some embodiments, the ester-basedsolvent system for the hole transporting layer ink composition comprisesan alkyl octanoate. This includes embodiment in which the ester-basedsolvent system comprises, consists of, or consists essentially of amixture of diethyl octanoate and octyl octanoate.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings, wherein like numeralsdenote like elements.

FIG. 1 is a block diagram that illustrates an organic light-emittingdiode (OLED) inkjet printing system.

FIG. 2 is a schematic illustration of a flat panel display comprising aplurality of OLEDs arranged in a matrix of pixel cells, each pixel cellbeing defined by a pixel bank.

FIG. 3(A) shows a microphotograph of the luminescence emitted from anOLED having an EML printed with an ink composition comprising anester-based solvent system of diethyl sebacate.

FIG. 3(B) is a black and white line drawing of the microphotograph inFIG. 3(A)

FIG. 3(C) shows the emission intensity distribution corresponding to themicrograph of FIG. 3(A).

FIG. 4(A) shows a microphotograph of the luminescence emitted from anOLED having an EML printed with an ink composition comprising anester-based solvent system of octyl octanoate.

FIG. 4(B) is a black and white line drawing of the microphotograph inFIG. 4(A)

FIG. 4(C) shows the emission intensity distribution corresponding to themicrograph of FIG. 4(A).

FIG. 5(A) shows a microphotograph of the luminescence emitted from anOLED having an EML printed with an ink composition comprising anester-based solvent system of a mixture of octyl octanoate, diallyphthalate and isononyl isononate.

FIG. 5(B) is a black and white line drawing of the microphotograph inFIG. 5(A)

FIG. 5(C) shows the emission intensity distribution corresponding to themicrograph of FIG. 5(A).

FIG. 6(A) is a graph showing the frequency response of the dropletvolume for an EML ink composition comprising an ester-based solventsystem of diethyl sebacate.

FIG. 6(B) is a graph showing the frequency response of the dropletvelocity for an EML ink composition comprising an ester-based solventsystem of diethyl sebacate.

FIG. 7(A) is a graph showing the frequency response of the dropletvolume for an HTL ink composition comprising an ester-based solventsystem of ethyl octanoate and octyl octanoate.

FIG. 7(B) is a graph showing the frequency response of the dropletvelocity for an HTL ink composition comprising an ester-based solventsystem of ethyl octanoate and octyl octanoate.

DETAILED DESCRIPTION

Ink compositions for forming active layers in an OLED are provided. Alsoprovided are methods of forming active layers of an OLED using the inkcompositions. The ink compositions are able to provide layers havinghighly uniform thicknesses and homogenous compositions. As a result,OLEDs fabricated using the ink compositions are able to provide highlyuniform light emission profiles.

The ink compositions comprise a solvent system in which a substantialmajority of the solvent is comprised of one or more ester compounds,referred to herein as ester-based solvent systems. For example, theester-based solvent systems may comprise at least 90 weight percent (wt.%) ester solvents based on the total weight of the solvents in thesystem. This includes embodiments in which the ester-based solventsystems comprise at least 99 wt. % ester solvents and further includesembodiments in which the ester-based solvent systems consist essentiallyof, or consist of, ester solvents. Alkyl sebacates, such as di-n-alkylsebacates, alkyl octanoates and mixtures of these with other esters areexamples of esters that that are well suited for use in the ester-basedsolvent systems.

The solvent systems solubilize the functional organic polymers and/orcompounds of an active layer in an OLED. The nature of these functionalmaterials will depend on the nature of the layer being formed. Forexample, in ink compositions useful for forming a light-emitting layer,organic electroluminescent materials will be dissolved in theester-based solvent system. Similarly, in ink compositions useful forforming a hole transporting layer, hole transporting materials will bedissolved in the ester-based solvent system. The ester-based solventsystem may be used, for example, in place of more conventional solvents,such as dodecyl benzene, that are presently used in printable inkformulations for OLEDs.

The ink compositions can be deposited using a variety of film-formingtechniques, including spin-coating, casting, thermal printing and inkjetprinting. Therefore, the workable properties of the ink compositionswill depend on the intended film-forming technique. Generally, for inkcompositions useful for inkjet printing applications, the surfacetension, viscosity and wetting properties of the ink compositions shouldbe tailored to allow the compositions to be dispensed through an inkjetprinting nozzle without drying onto or clogging the nozzle at thetemperature used for printing (e.g., room temperature; ˜23° C.). Morespecifically, it has been discovered that the ester-based solventsystems having a boiling point of at least about 300° C. (for example, aboiling point in the range from about 300° C. to about 330° C.) and asurface tension at 23° C. in the range from about 25 dyne/cm to about 33dyne/cm are able to provide inkjet printed layers having highly uniformthicknesses and compositional homogeneity across the width of the layer.(For the purposes of this disclosure, the recited boiling points referto boiling points at atmospheric pressure, unless otherwise indicated.)

Inkjet printers suitable for printing the present ink compositions arecommercially available and can include drop-on-demand printheads,available from, for example, Fujifilm Dimatix (Lebanon, N.H.), TridentInternational (Brookfield, Conn.), Epson (Torrance, Calif.), HitachiData systems Corporation (Santa Clara, Calif.), Xaar PLC (Cambridge,United Kingdom), and Idanit Technologies, Limited (Rishon Le Zion,Isreal). For example, the Dimatix Materials Printer DMP-3000 may beused.

Light-emitting layers (EMLs) and hole transporting layers (HTLs) areexamples of the types of active OLED layers that can be printed usingthe present ink compositions. For example, ink compositions for an HTLcomprise an a hole transporting material, typically a semi-conducting,crosslinkable polymer, dissolved in an ester-based solvent system havinga boiling point of at least 300° C. and a surface tension at 23° C. inthe range from about 25 dyne/cm to about 32 dyne/cm. Some embodiments ofthe ester-based solvent systems for HTL ink compositions comprise,consist essentially of, or consist of one or more alkyl octanoates, suchas octyl octanoate, diethyl octanoate or a mixture of thereof. Aspecific embodiment of one such ink composition comprises a mixture ofdiethyl octanoate and octyl octanoate in a weight ratio in the rangefrom about 40:60 to about 60:40. This includes embodiments in which theweight ratio is in the range from about 45:55 to about 55:45 and furtherincludes embodiments in which the weight ratio is about 50:50.

Examples of the polymers that may be included in the HTL inkcompositions include polyvinyl carbazoles or derivatives thereof,polysilanes or derivatives thereof, polysiloxane derivatives having anaromatic amine at the side chain or the main chain, pyrazolinederivatives, arylamine derivatives, stilbene derivatives,triphenyldiamine derivatives, polyaniline or derivatives thereof,polythiophene or derivatives thereof, polyarylamines or derivativesthereof, polypyrroles or derivatives thereof, poly(p-phenylenevinylene)or derivatives thereof, or poly(2,5 thienylene vinylene) or derivativesthereof.

The concentration of hole transporting material in the HTL inkcomposition should be selected to render the ink composition suitablefor the film-forming technique with which it will be applied. By way ofillustration only, for an HTL ink composition useful in inkjet printingapplications, the concentration of hole transporting material may be inthe range from about 0.1 to about 5 wt. % (e.g., from about 0.5 to about2 wt. %) based on the combined weight of the hole transporting materialand the ester-based solvent system. Desirably, the hole transportingmaterials are sufficiently soluble in the solvents of the solvent systemto provide an ink composition having a viscosity of 100 cPs or lower(e.g., 50 cPs or lower) at a hole transporting material concentration inthe ink composition of up to 3 wt. %, or even higher, at the printingtemperature.

Ink compositions for an EML comprise a light-emitting material dissolvedin an ester-based solvent system having a boiling point of at least 300°C. and a surface tension at 23° C. in the range from about 26 dyne/cm toabout 33 dyne/cm. Some embodiments of the ester-based solvent systemsfor EML ink compositions comprise, consist essentially of, or consist ofan alkyl sebacate, such as diethyl sebacate, an alkyl octanoate, such asoctyl octanoate, or a mixture comprising one or more alkyl sebacates oralkyl octanoates with another ester solvent, such as a phthalate. It hasbeen discovered that alkyl sebacates, such as diethyl sebacate, areparticularly useful in the ester-based solvent systems for EML inksbecause their wetting behavior, as measured by contact angle, isindependent of, or substantially independent of the degree ofcrosslinking in an underlying HTL comprising a crosslinkable holetransporting polymer material. This is important because it allows inkcompositions comprising diethyl sebacate to form EMLs having highlyuniform and reproducible layer thicknesses on HTL substrates, even whenthe crosslinking in those HTLs is incomplete or differs from substrateto substrate.

A specific embodiment of one such ink composition comprises a mixture ofoctyl octanoate with diallyl phthalate. The octyl octanoate and diallyphthalate may be present, for example, in a weight ratio in the rangefrom about 60:40 to about 80:20. This includes embodiments in which theweight ratio is in the range from about 65:35 to about 75:25 and furtherincludes embodiments in which the weight ratio is about 70:30.

Another embodiment of an EML ink composition comprises a mixture ofoctyl octanoate, diallyl phthalate and isononyl isononate. The weightratio of the octyl octanoate to the combined weight of the diallylphthalate and isononyl isononate in this ink composition may be in therange from about 50:50 to about 60:40, for example. The weight ratio ofthe dially phthalate to isononyl isononate in this composition may be inthe range from about 60:40 to about 70:30, for example. By way ofillustration only, one such EML ink composition comprises a mixture ofoctyl octanoate, diallyl phthalate and isononyl isononate in a weightratio of about 55:30:15.

The light-emitting materials of the EML ink compositions generallycomprise an organic polymer, small molecule or dendrimer that is itselfa light-emitting material or that provides a light-emitting material incombination with one or more additional dopants, including metal complexdopants. For example, in some embodiments, the light-emitting materialscomprise a polymeric host matrix and one or more dopants dispersedwithin the host matrix. Examples of organic polymer host materials thatmay be included in the EML ink composition include polyparaphenylenevinylene derivatives, polythiophene derivatives, polyparaphenylenederivatives, polysilane derivatives, polyacetylene derivatives,polyfluorene derivatives, and polyvinyl carbazole derivatives. In otherembodiments, the host matrix material may be a small molecule. Metalcomplexes of 8-hydroxyquinoline and similar derivatives constitute oneclass of useful host materials and are particularly suitable for lightemission at wavelengths longer than 500 nm. Benzazole derivativesconstitute another class of useful host materials and are particularlyuseful for light emission wavelengths above 400 nm. The host materialcan also include an anthracene derivative having substitutions in the 9and 10 positions, for example derivatives of 9,10-diarylanthracenes.

Examples of the dopant materials that may be included in the inkcompositions include perylene derivatives, coumarin derivatives, rubrenederivatives, quinacridone derivatives, squarylium derivatives, porphyrinderivatives, styryl pigments, tetracene derivatives, pyrazolonederivatives, decacyclene, and phenoxazon, cyclopendamine derivatives,tetraphenyl butadiene derivative compounds, triphenyl amine derivatives,oxadiazole derivatives, pyrazoloquinoline derivatives, distyrylbenzenederivatives, distyrylarylene derivatives, pyrrole derivatives, thiophenering compounds, pyridine ring compounds, perynone derivatives,oligothiophene derivatives, oxa-diazole dimers, pyrazoline dimers,quinacridone derivatives, and coumarin derivatives. Examples of metalcomplex dopants that may be included in the EML ink compositions includemetal complexes having as a central metal, AI, Zn, Be, a rare-earthmetal such as Tb, Eu, and Dy, or the like and having as a ligand, astructure of oxadiazole, thiadiazole, phenylpyridine,phenylbenzimidazole, quinoline, or the like, for example, metalcomplexes such as iridium complexes and platinum complexes that emitlight from the triplet excited state, benzoquinolinole berylliumcomplexes, benzoxazolyl zinc complexes, benzothiazole zinc complexes,azomethyl zinc complexes, porphyrin zinc complexes, and europiumcomplexes.

The concentration of light-emitting material in the EML ink compositionshould be selected to render the ink composition suitable for thefilm-forming technique with which it will be applied. By way ofillustration only, for an EML ink composition useful in inkjet printingapplications, the concentration of light-emitting material may be in therange from about 0.1 to about 5 wt. % (e.g., from about 0.5 to about 2wt. %) based on the combined weight of the light-emitting material andthe ester-based solvent system.

Methods for forming an active layer of an OLED using the present inkcompositions comprise the steps of forming a layer of the inkcomposition on a substrate using, for example, an OLED inkjet printingsystem, and allowing the solvents of the ester-based solvent system toevaporate. The step of allowing the solvents of the ester-based solventsystem to evaporate may be facilitated by subjecting the layer of theink composition to reduced pressure, that is—exposing it to a vacuum, byexposing the layer of the ink composition to elevated temperatures, or acombination of the two. Optionally, the layer subsequently may be bakedin order to further remove solvent from the layer and/or to facilitatethe crosslinking of any crosslinkable polymers in the ink formulation.Thus, a baking step may be used after the initial formation and an HTLcomprising a crosslinkable polymer in order to crosslink said polymer,rendering it insoluble in the solvent system of a subsequently depositedlayer.

Various embodiments of an OLED inkjet printing system can be comprisedof several devices and apparatuses, which allow the reliable placementof drop of an ink composition onto specific locations on a substrate.These devices and apparatuses can include, but are not limited to, aprinthead, ink delivery system, motion system, substrate loading andunloading system, and printhead maintenance system. A printhead includesat least one inkjet head, with at least one orifice capable of ejectingdrops of the ink composition at a controlled rate, velocity, and size.The printhead can be fed by an ink composition supply system thatprovides the ink composition to the printhead. Printing requiresrelative motion between the printhead and the substrate. This can beaccomplished with a motion system, typically a gantry or split axis XYZsystem. Either the printhead can move over a stationary substrate(gantry style), or both the printhead and substrate can move, in thecase of a split axis configuration. In another embodiment, the printstation can be fixed, and the substrate can move in the X and Y axesrelative to the printheads, with Z axis motion provided either at thesubstrate or the printhead. As the printheads move relative to thesubstrate, drops of ink composition are ejected at the correct time tobe deposited in the desired location on the substrate. The substrate canbe inserted and removed from the printer using a substrate loading andunloading system. Depending on the printer configuration, this can beaccomplished with a mechanical conveyor, a substrate floatation table,or a robot with end effector.

A printhead maintenance system can be comprised of several subsystemsthat allow for such maintenance tasks, such as, but not limited by, dropvolume measurement, elimination of excess ink from a printhead nozzleplate surface, and priming for ejecting ink into a waste basin.

FIG. 1 is a block diagram that illustrates an OLED inkjet printingsystem 100 that can be used to print the present ink compositions.System 100 includes printhead device 110, ink delivery system 120,motion system 130, substrate loading and unloading system 140, printheadmaintenance system 150, processor 160, drop measurement device 170, andnozzle tuning device 180. Printhead device 110 includes at least oneinkjet head, with at least one orifice capable of ejecting drops of anink composition at a controlled velocity, and size. The inkjet head isfed by an ink supply system that provides the ink composition to theinkjet head. In various embodiments, the inkjet or printhead includesmultiple orifices or nozzles that allow ink to be dispensed at more thanone location on the substrate or panel at the same time. For example,ink composition is supplied to printhead device 110 using ink deliverysystem 120. Printhead maintenance system 150 can include severalsubsystems that allow for such maintenance tasks as drop volumemeasurement, wiping of the inkjet nozzle surface, priming for ejectingan ink composition into a waste basin.

Processor 160 is used to control and send and/or receive data to and/orfrom printhead 110, ink delivery system 120, motion system 130,substrate loading and unloading system 140, and printhead maintenancesystem 150. Processor 160 can be a computer system, such as the computersystem shown in FIG. 1, a microcontroller, an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), or anelectronic circuit capable of sending and receiving control and datainformation and capable of executing instructions. Processor 160 can beone electronic circuit or multiple electronic circuits distributed amongprinthead 110, ink delivery system 120, motion system 130, substrateloading and unloading system 140, and printhead maintenance system 150that can communicate with each other, for example.

The final product of the printing, drying and optional baking process isa layer of material having a highly uniform thickness and composition.For example, layers having a thickness variation no greater than 10%across the entire width of the layer are possible. The thickness acrossthe layer can be measured using metrology tools, such as a styluscontact profilometer or an interferometer microscope. Suitableinterferometers for optical interferometry are commercially availablefrom Zygo instrumentation.

The ink compositions can be used to print active layers directly in amulti-layered OLED architecture. A typical OLED comprises a supportsubstrate, an anode, a cathode and a light-emitting layer disposed inbetween the anode and cathode. Other layers that may be present in thedevice, include a layer of conductive hole injection material (HIL)provided between the anode and the light-emitting layer to assist withthe injection of holes from the anode to the light-emitting layer, thehole transporting layer provided between the HIL (if present) and thelight-emitting layer to assist with the transport of holes to thelight-emitting layer, and an electron transporting layer (ETL) disposedbetween the EML and the cathode. The substrate is generally atransparent glass or plastic substrate and at least one of the anode andcathode is generally transparent in order to facilitate the transmissionof the light emitted from the EML.

The HIL layer can also be formed using an ink composition comprising anester-based solvent system. Such ink formulations will further compriseHIL materials comprising a conducting organic or inorganic material,which is typically doped. Examples of such materials includephenylamines, starburst-type amines, phthalocyanines, polyaniline, andpolythiophene derivatives.

In these multi-layered architectures, one or more layers may be formedusing ink compositions comprising the present ester-based solventsystems, while other layers may be formed using other types of solventsystems. Moreover, one or more layers may be formed via inkjet printing,while other layers may be deposited using other film-forming techniques.Typically, the various layers will be formed within one or more pixelcells defined by their corresponding pixel banks. Each cell comprises afloor and a wall that defines the perimeter of the cell. The surfaceswith a cell optionally may be coated with a surface-modifying coating,such as a surfactant in order to keep deposited ink composition dropletsfrom flooding out of the cells. However, in some embodiments, suchsurfactants are absent, as they may quench the luminescence of thelight-emitting layer.

FIG. 2 is a schematic illustration of a flat panel display comprising aplurality of OLEDs arranged in a matrix of pixel cells defined by amatrix of pixel banks. FIG. 2 depicts an expanded view 220 of an area ofpanel 200, showing the arrangement 230 of a plurality of pixel cells,including a red light-emitting pixel cell 232, a green light-emittingpixel cell 234 and blue light-emitting pixel cell 236. Additionally,integrated circuitry 238 can be formed on a flat panel display substrateso that the circuitry is adjacent to each pixel cell for the purpose ofapplying voltage to each pixel in a controlled fashion during use. Pixelcell size, shape, and aspect ratios can vary depending on, for example,but not limited by, the resolution desired. For example, a pixel celldensity of 100 ppi can be sufficient for a panel used for a computerdisplay, where for high resolution of, for example of between about 300ppi to about 450 ppi, can result in various pixel cell designs amenableto the effective packing of higher pixel density on a substrate surface.

Example

The following example illustrates the use of ink compositions comprisingester-based solvent system to print an HTL and an EML in an OLED devicearchitecture.

Materials and Methods.

Preparation of HTL Ink Composition:

A 30 ml amber vial and stir bar were cleaned and transferred to a glovebox. A quantity of 0.12 g of hole transporting polymer powder wasmeasured out in the inert environment of the glove box. An ester-basedsolvent system composed of 29.88 g of a mixture of distilled anddegassed diethyl octanoate and octyl octanoate was prepared at a weightratio of 1:1 in the vial. The prepared hole transporting solids werethen added to the vial and mixed with the ester-based solvent systemusing the stir bar. The resulting mixture was stirred on a hot plate setat 60° C. until the solid powders were fully dissolved. The vial wasthen removed from the hot plate and allowed to cool to room temperature.The resulting solution was filtered through a 0.2 μm PTFE filter into anamber 100 mL glass bottle. A quantity of 2 mL of the solution was setaside for viscosity and surface tension measurements. Then the bottlewas sealed, removed from the glove box and set aside for printing.

Preparation of EML Ink Composition:

30 ml amber vials and stir bars were cleaned and transferred to a glovebox. A quantity of 0.3 g of organic electroluminescent material wasmeasured out in the inert environment of the glove box. Quantities of29.7 g of three different ester-based solvent systems were prepared inseparate vials. The ester solvent of the first system was distilled anddegassed diethyl sebacate; the ester solvent of the second system wasoctyl octanoate; and the ester solvents of the third system were amixture of octyl octanoate, diallyl phthalate and isononyl isononate ina weight ratio of about 5.5:3:1.5. The prepared organicelectroluminescent solids were added to the vials and mixed with theester-based solvent systems using the stir bars. The resulting mixtureswere stirred on a hot plate set at 150° C. until the solid powders werefully dissolved. The vials were then removed from the hot plate andallowed to cool to room temperature. The resulting solutions werefiltered through a 0.2 PTFE filter into an amber 100 mL glass bottle. Aquantity of 2 mL of each solution was set aside for viscosity andsurface tension measurements. Then the bottles were sealed, removed fromthe glove box and set aside until for printing.

Viscosity and Surface Tension Measurements:

Viscosity measurements were carried out using a DV-I Prime Brookfieldrheometer. Surface tension was measured with a SITA bubble pressuretensiometer.

Using these methods, the viscosity of the HTL ink composition wasdetermined to be 3.71±0.1 cPs and its surface tension was determined tobe 27.3±0.3 dyne/cm.

The viscosity of the EML ink composition comprising the firstester-based solvent system was determined to be 5.7±0.2 cPs and itssurface tension was determined to be 31.5±0.1 dyne/cm. The viscosity ofthe EML ink composition comprising the second ester-based solvent systemwas determined to be 3.8±0.1 cPs and its surface tension was determinedto be 27.5±0.1 dyne/cm. The viscosity of the EML ink compositioncomprising the third ester-based solvent system was determined to be5.6±0.2 cPs and its surface tension was determined to be 29.1±0.1dyne/cm.

OLED Fabrication and Printing:

In this example, the HTL and EML layers of the OLED are inkjet printedusing ink compositions comprising ester-based solvent systems, asdescribed above. The HIL device layer, a commercially availableformulation, was also ink-jet printed. The other device layers areformed by other means, as described in greater detail below.

For the inkjet printed HTL and EML layers, the ink composition is loadedinto the ink cartridge for use with an inkjet printer, such as a DimatixMaterials Printer DMP-3000, available from Fujifilm. The ink cartridgeis then placed in the printer and the substrate positioned below theprint head. A waveform for firing the ink is developed and the pulsetimes and voltages are adjusted and optimized to establish a stablejetting range. For example, ink jet tests can be conducted to assess theprint performance of the ink compositions by examining the effects ofchanging the frequency on droplet volume and velocity. Examples ofresults from this type of frequency response test, after optimizing thepulse times and voltages, are illustrated in the graphs of FIGS. 6(A)and 6(B) which show the frequency response of droplet volume andvelocity, respectively, for the EML ink composition comprising diethylsebacate. FIGS. 7(A) and 7(B) show the frequency response of dropletvolume and velocity, respectively, for the HTL ink composition comprisedof the mixture of diethyl octanoate and octyl octanoate.

The substrate of the OLED is glass, with a thickness of 0.7 mm, on whichan anode of 60 nm ITO (indium tin oxide) is patterned. A bank material(also known as a pixel definition layer) is then patterned over the ITO,forming a well into which the inkjet printed layers are deposited. AnHIL layer composed of PEDOT:PSS(poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)) is then ink-jetprinted into the well and dried under vacuum and baked at an elevatedtemperature in order to remove solvent from the layer. The HTL layer isthen inkjet printed onto the HIL layer according to the processdescribed above, followed by drying under vacuum and baking at anelevated temperature to remove solvent and induce the crosslinking inthe crosslinkable polymer. Then the EML layer is inkjet printed onto theHTL layer according to the process described above, followed by dryingunder vacuum and baking at an elevated temperature to remove solvent. Acathode layer is then applied by vacuum thermal evaporation. An ETLlayer followed by a cathode layer are then applied by vacuum thermalevaporation. The cathode layer is composed of 100 nm of aluminum.

Characterization

Once the OLEDs were fabricated, the uniformity of theirelectroluminescence was investigated by applying an electrical currentacross each diode and imaging the light emission. The results of areshown in FIGS. 3-5. The panel (A) in each figure is a microphotographshowing an image of the luminescence of each OLED, panel (B) in eachfigure is a black and white line rendering of the microphotographs, andpanel (C) in each figure is the corresponding electroluminescenceintensity distribution across the width of each OLED. In FIGS. 3(C),4(C) and 5(C) the data are normalized to the brightest point ofluminescence across the length of the pixel. The emission from the OLEDis represented by regions I and II in the B panels of FIGS. 3-5. The Cpanels of FIGS. 3-5 show the uniformity distribution of the OLEDemission across the width of the pixel, including regions I and II.Typically, the emission is slightly higher in region II compared toregion I. The pixel bank defines the active area of the OLED pixel andis located at the outer edge of region II.

The uniformity of the luminescence from a pixel can be quantified by thecoefficient of variation in the luminescence (CV %), which is defined asits ((standard deviation)/mean)*100). In some embodiments, the presentester based solvent systems enable the fabrication of OLED pixels havinga CV % of no greater than 20%. This includes embodiments of OLED pixelshaving a CV % of no greater than 16%, no greater than 12% and no greaterthan 10%. By way of illustration only, some embodiments of the OLEDpixels provide a CV % in the range from about 2 to about 20%. Thisincludes embodiments that provide a CV % in the range from about 5 toabout 15%.

By way of illustration, the CV % of the OLED pixels fabricated inaccordance with this example were measured as follows. The luminescenceemission was measured across a 2D region of interest (ROI), representedas region I in FIGS. 3(A), 3(B), 4(A), 4(B), 5(A) and 5(B), the standarddeviation and mean for the emission from the ROI was then determined foreach OLED and the CV % was calculated. The results of the calculationsare provided in Table 1, below.

TABLE 1 CV % for OLED Pixels Ester-Based Solvent System ((StandardDeviation/Mean) * 100) Used in EML Ink Composition (CV % forLuminescence) Diethyl Sebacate 11.38 Octyl Octanoate 9.82 OctylOctanoate, Diallyl 15.79 Phthalate and Isononyl Isononate Mixture

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more.”

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. An ink composition comprising a conductive holeinjection material dissolved in an ester-based solvent system, theester-based solvent system having a boiling point of at least 300° C.and a surface tension at 23° C. in the range from about 25 dyne/cm toabout 33 dyne/cm and comprising at least one ester, wherein at least 90weight percent of the ester-based solvent system comprises the at leastone ester, and wherein the at least one ester comprises an alkyloctanoate.
 2. The ink composition of claim 1, wherein the ester-basedsolvent system comprises at least 99 weight percent esters.
 3. The inkcomposition of claim 1, wherein the ester-based solvent system comprisesat least 90 weight percent of the alkyl octanoate.
 4. The inkcomposition of claim 1, wherein the ester-based solvent system furthercomprises an alkyl sebacate.
 5. The ink composition of claim 1, whereinthe ester-based solvent system comprises two or more alkyl octanoates.6. The ink composition of claim 5, wherein the ester-based solventsystem comprises diethyl octanoate and octyl octanoate.
 7. The inkcomposition of claim 5, wherein the ester-based solvent system consistsessentially of diethyl octanoate and octyl octanoate.
 8. The inkcomposition of claim 1, wherein the hole injection material is selectedfrom phenylamines, starburst-type amines, phthalocyanines, andpolyaniline.
 9. A method of forming a hole injection layer for anorganic light-emitting diode, the method comprising: forming a layer ofan ink composition in a pixel cell of an organic light-emitting diodepixel bank, the ink composition comprising a conductive hole injectionmaterial dissolved in an ester-based solvent system, the ester-basedsolvent system having a boiling point of at least 300° C. and a surfacetension at 23° C. in the range from about 25 dyne/cm to about 33 dyne/cmand comprising at least one ester, wherein at least 90 weight percent ofthe ester-based solvent system comprises the at least one ester, andwherein the at least one ester comprises an alkyl octanoate; andallowing the solvent from the solvent system to evaporate, whereby thehole injection layer is formed.
 10. The method of claim 9, whereinforming a layer of an ink composition in a pixel cell of an organiclight-emitting diode pixel bank comprising inkjet printing the inkcomposition into the pixel cell of the organic light-emitting diodepixel bank.
 11. The method of claim 9, wherein the ester-based solventsystem comprises at least 99 weight percent esters.
 12. The method ofclaim 9, wherein the ester-based solvent system comprises two or morealkyl octanoates.
 13. The method of claim 12, wherein the ester-basedsolvent system comprises diethyl octanoate and octyl octanoate.
 14. Themethod of claim 9, wherein the hole injection material is selected fromphenylamines, starburst-type amines, phthalocyanines, and polyaniline.